RF power amplifiers are used in a variety of applications such as base stations for wireless communication systems etc. RF power amplifiers are designed to provide linear operation without distortion. The signals amplified by the RF power amplifiers often include signals that have a high frequency modulated carrier having frequencies in the 400 Megahertz (MHz) to 4 Gigahertz (GHz) range. The baseband signal that modulates the carrier is typically at a relatively lower frequency and, depending on the application, can be up to 300 MHz or higher.
A device package for an RF power amplifier can include a transistor die (e.g., MOSFET (metal-oxide semiconductor field-effect transistor), LDMOS (laterally-diffused metal-oxide semiconductor), HEMT (high electron mobility transistor) along with an input and output impedance matching circuit incorporated therein. The input and output impedance matching circuits typically include LC networks that provide at least a portion of an impedance matching circuit that is configured to match the impedance of the transistor die to a fixed value. These input and output impedance matching circuits are used to match the relatively low characteristic impedances of RF transistors (e.g., impedances (e.g., around 2 ohms or less for high power devices), to a fixed impedance value (e.g., 50 ohms). These input and output impedance matching circuits are frequency selective and introduce impedance dispersion versus frequency, which results in band limited power amplifier operations. The impedances presented to the devices in the higher order harmonic frequency ranges significantly impact the performance of the amplifier, and in particular the efficiency of the amplifier. In conventional impedance matching networks, impedance transformation is typically satisfactory only in a limited frequency range. For example, an optimized input matching network requires a frequency response in terms of source reflection coefficient at the second harmonic frequency to present a certain range of phases in order to obtain consistent performances versus frequency with minimal variation. Outside of this range of phases, efficiency is dramatically degraded.
Conventionally, impedance matching networks are tuned primarily at the center frequency of the fundamental frequency range. The phase of the second harmonic reflection coefficient is implicitly determined without an explicit design parameter. Therefore, it is difficult to be applied to multiple device characteristics. One technique for optimizing amplifier efficiency involves introducing a resonant circuit (e.g., an LC resonator) that is configured to provide a second harmonic short (180°) at the input of the device. In such a design, the efficiency performance is close to the maximum when the second harmonic phase is close to the short (180° in phase). However, in such a circuit, the second harmonic frequency response is highly dispersive. Thus, while the second harmonic short improves narrowband performance, this topology suffers from decreased broadband performance.